The university-developed GapSense system could help prevent interference between Wi-Fi and other networks

Researchers at the University of Michigan have invented a way for different wireless networks crammed into the same space to say "excuse me" to one another.

Wi-Fi shares a frequency band with the popular Bluetooth and ZigBee systems, and all are often found in the same places together. But it's hard to prevent interference among the three technologies because they can't signal each other to coordinate the use of the spectrum. In addition, different generations of Wi-Fi sometimes fail to exchange coordination signals because they use wider or narrower radio bands. Both problems can slow down networks and break connections.

Michigan computer science professor Kang Shin and graduate student Xinyu Zhang, now an assistant professor at the University of Wisconsin, set out to tackle this problem in 2011. Last July, they invented GapSense, software that lets Wi-Fi, Bluetooth and ZigBee all send special energy pulses that can be used as traffic-control messages. GapSense is ready to implement in devices and access points if a standards body or a critical mass of vendors gets behind it, Shin said.

Wi-Fi LANs are a data lifeline for phones, tablets and PCs in countless homes, offices and public places. Bluetooth is a slower but less power-hungry protocol typically used in place of cords to connect peripherals, and ZigBee is an even lower powered system found in devices for home automation, health care and other purposes.

Each of the three wireless protocols has a mechanism for devices to coordinate the use of airtime, but they all are different from one another, Shin said.

"They can't really speak the same language and understand each other at all," Shin said.

Each also uses CSMA (carrier sense multiple access), a mechanism that instructs radios to hold off on transmissions if the airwaves are being used, but that system doesn't always prevent interference, he said.

The main problem is Wi-Fi stepping on the toes of Bluetooth and ZigBee. Sometimes this happens just because it acts faster than other networks. For example, a Wi-Fi device using CSMA may not sense any danger of a collision with another transmission even though a nearby ZigBee device is about to start transmitting. That's because ZigBee takes 16 times as long as Wi-Fi to emerge from idle mode and get the packets moving, Shin said.

Changing ZigBee's performance to help it keep up with its Wi-Fi neighbors would defeat the purpose of ZigBee, which is to transmit and receive small amounts of data with very low power consumption and long battery life, Shin said.

Wi-Fi devices can even fail to communicate among themselves on dividing up resources. Successive generations of the Wi-Fi standard have allowed for larger chunks of spectrum in order to achieve higher speeds. As a result, if an 802.11b device using just 10MHz of bandwidth tries to tell the rest of a Wi-Fi network that it has packets to send, an 802.11n device that's using 40MHz may not get that signal, Shin said. The 802.11b device then becomes a "hidden terminal," Shin said. As a result, packets from the two devices may collide.

To get all these different devices to coordinate their use of spectrum, Shin and Zhang devised a totally new communication method. GapSense uses a series of energy pulses separated by gaps. The length of the gaps between pulses can be used to distinguish different types of messages, such as instructions to back off on transmissions until the coast is clear. The signals can be sent at the start of a communication or between packets.

GapSense might noticeably improve the experience of using Wi-Fi, Bluetooth and ZigBee. Network collisions can slow down networks and even cause broken connections or dropped calls. When Shin and Zhang tested wireless networks in a simulated office environment with moderate Wi-Fi traffic, they found a 45 percent rate of collisions between ZigBee and Wi-Fi. Using GapSense slashed that collision rate to 8 percent. Their tests of the "hidden terminal" problem showed a 40 percent collision rate, and GapSense reduced that nearly to zero, according to a press release.

One other possible use of GapSense is to let Wi-Fi devices stay alert with less power drain. The way Wi-Fi works now, idle receivers typically have to listen to an access point to be prepared for incoming traffic. With GapSense, the access point can send a series of repeated pulses and gaps that a receiver can recognize while running at a very low clock rate, Shin said. Without fully emerging from idle, the receiver can determine from the repeated messages that the access point is trying to send it data. This feature could reduce energy consumption of a Wi-Fi device by 44 percent, according to Shin.

Implementing GapSense would involve updating the firmware and device drivers of both devices and Wi-Fi access points. Most manufacturers would not do this for devices already in the field, so the technology will probably have to wait for hardware products to be refreshed, according to Shin.

A patent on the technology is pending. The ideal way to proliferate the technology would be through a formal standard, but even without that, it could become widely embraced if two or more major vendors license it, Shin said.